1. Field of the Invention
This invention relates to piezoelectric materials and especially relates to providing improved electrodes on flexible and conformable ferroelectric and piezoelectric composites ("piezo-composites") and thick piezoelectric polymer films ("piezo-films") to be used in various electromechanical transducer and sensor applications.
2. Review of the Prior Art
A variety of electro-mechanical transducers, such as hydrophones, noise sensors, vibration sensors, and pressure and stress sensors, depend on the piezoelectric phenomenon exhibited by certain piezoelectric crystals, polarized ceramics, and polarized polymers. Hydrophones, having useful sensitivity and capacitance and able to withstand large hydrostatic pressures, have been made by using piezoelectric polyvinylidene fluoride film as the transduction material. The film is mounted on a stiff circular plate (disk) for strength under hydrostatic pressure; strains developed on the disk surface by incident acoustic pressure are exploited to excite the film, and generate electrical voltage as described by T. D. Sullivan and J. M. Powers, J. Acoust. Soc. Am., Vol. 63, No. 5, May 1978.
Polyvinylidene fluoride (PVDF) in film form has especially been investigated as the sensing material in advanced passive sonar arrays. Its piezoelectric properties are sufficiently high to compete with the established piezoelectric ceramic ("piezo-ceramic") sensor elements, and its conformability and large size make it suitable for large arrays which operate in the hydrostatic receiver mode. PVDF hydrophones have been demonstrated which are free of hydrostatic resonances over a broad frequency range.
As discussed by R. H. Tancrell et al in the Proceedings of the IEE Ultrasonics Symposium, Catalog No. 85CH2209-5, pages 624-629 (1985), the Figure of Merit, FM, characterizing the hydrophone element sensitivity, is given by the formula: EQU FM=(d.sub.h).sup.2 (t)(A)/.epsilon.'=(d.sub.h)(g.sub.h)(U)
where d.sub.h and g.sub.h are the piezoelectric charge and voltage coefficient, respectively, under hydrostatic pressure, t is the thickness, A is the area, U is the volume of the hydrophone element, and .epsilon.' is its permittivity. Subjecting a sample to hydrostatic pressure causes the simultaneous application of three orthogonal force components, F.sub.1, F.sub.2, F.sub.3, according to the equation: ##EQU1## F.sub.1 being applied in the stretch direction and in the plane of the polymer, F.sub.2 being applied transversely to the stretch direction and also in the plane of the polymer, and F.sub.3 being applied perpendicularly to the plane of the polymer. D.sub.33 is the piezoelectric charge coefficient in the vertical (thickness) direction, and d.sub.31 and d.sub.32 are the respective piezoelectric charge coefficients in the transverse plane - d.sub.31 referring to the stretch direction and d.sub.32 transversely to it.
A layer of metal deposited on the surface was found to have a dramatic effect on PVDF's elastic and piezoelectric properties, especially the transverse piezo-coefficient, d.sub.31, due to the softness of the PVDF polymer relative to that of the metal. Although metallic or non-metallic stiffening layers are often included in actual piezo-ceramic hydrophone assemblies to eliminate undesirable resonance frequencies, the properties of the piezo-ceramic do not change upon coating with metal because the latter is not nearly as stiff as the ceramic itself.
During the last decade composite materials have been developed that are piezoelectric as well as flexible. Their composition and properties have been described in reviews, such as the one by R. E. Newnham, L. J. Bowen, K. A. Klicker and L. E. Cross, titled "Composite Piezoelectric Transducers" and published in Materials in Engineering, Vol. 2, December 1980, pp. 93-106. The most common of these are the so-called 0-3 connectivity composites which basically consist of two solid phases--a piezoelectrically active ceramic powder phase and a piezoelectrically inactive but highly coherent polymeric phase, the powder phase being dispersed uniformly in the polymeric phase. The mechanical properties of the 0-3 composites, such as stiffness, are about the same as those of the PVDF, but their piezoelectric and dielectric properties can often be significantly higher.
Both the PVDF and the piezo-composite are suitable for hydrophones, sonobuoys, and sonar arrays that operate in the listening mode, but both exhibit a serious inherent shortcoming in that conductive electrodes can be neither tightly adhered nor strongly and reliably connected electrically thereto.
Currently two approaches to accomplish the electroding and the bonding of electrical leads to the electrodes are being practiced. The first one involves the deposition of a thin layer of a highly electroconductive metal, such as aluminum, copper, nickel, or silver, by evaporation, sputtering, electron beam, or similar techniques onto the appropriate surfaces of the PVDF or the piezo-composite element, followed by soldering, gluing, or otherwise attaching the electrical leads to the metal. The other approach involves procuring a conductive plastic which can be properly bonded to the piezo-composite or the PVDF surfaces to function as the electrode. In this case, the electric leads cannot be soldered because none of the normally known solders will bond to the conductive plastic electrodes, and one resorts to attaching the metal leads to such electrodes by adhesive bonding with electrically conductive adhesives. Both of these approaches have known shortcomings.
In the first case, the as-deposited metal layer electrode is inherently incompatible with the underlying polymer base substrate, so that the latter must be given a demanding prior cleaning and/or surface treatment in order to achieve adequate adhesion. Furthermore, since the thermal expansion coefficients of the metal and the piezoelectric substrate differ greatly, the heat generated during the subsequent soldering of the leads often causes excessive thermal stresses which debond the electrode. Even if an adequate electroding and lead bonding is achieved, the metal electrode, because of its thinness, will have a relatively high electrical series resistance which in use increases its thermal (Johnson) noise and response lag (RC time constant), thereby lowering its electrical performance and frequency response.
In the second case, a satisfactory bonding between the conductive plastic electrode and the piezoelectric polymer base substrate is normally achievable, but the problem of securing the leads to this kind of electrode remains, because it is as difficult to achieve solderability to the conductive electrode as it is to the piezo-composite or the PVDF.
A means allowing the soldering of metallic leads to the above substrates quickly, reliably, and inexpensively, by modification of the conductive electrodes, is badly needed. The prior patent art that deals with the electroding of piezoelectric materials was found to refer to devices or assemblies having only piezo-ceramic as the active material.
In the prior art as disclosed in U.S. Pat. No. 2,423,922, a ribbon or strip of metallic foil, having a coating of material such as wax or a synthetic plastic applied to its opposite faces, has been used for joining together the pre-electroded faces of a pair of crystal sections with colloidal graphite or silver metal by heat and pressure to form a transducer. The ribbon may be made of silver or the like and have integral struck-out projections or teeth in order to establish conductive connection between the foil and substantially the entire electroded surface of each crystal section.
A special piezoelectric transducer called a "bender" or "bimorph", in the form of a flat disk, has been described in U.S. Pat. No. 3,629,625. This flat disk includes a pair of circular ceramic sheets which are precisely separated by a piezoelectrically inactive center vane which is corrugated, the apices of the corrugations being cemented to the sheets. Since the center vane itself provides a conductive path between the electrodes, nonconductive epoxy (which is stronger than conductive epoxy) is used to cement the electrodes and the ceramic sheets to the center vane.
In U.S. Pat. No. 4,078,160, a woven mesh of conductively coated polymeric filaments is described as affixed between a thin piezo-ceramic electroded disc, which is deformable in response to an applied electronic signal, and a second disc, which is identical to the first one and may also be electrically deformable. The mesh serves as a center vane between the two elements and allows the bender elements to move relative to each other. The bender is supported by peripheral tabs which extend from the mesh and are soldered to a terminal, providing electrical contact between and to the outer faces of the piezoelectric disk. The filaments are a stiff polyester and are coated with a metal such as nickel so that the threads are embedded in the metal which is fused at cross-over points. This electrically conducting mesh is further coated with an uncured solid epoxy. The mesh and two transducer elements are assembled together, and heat and pressure are applied to melt and then cure the epoxy, causing the conductive mesh material to make contact with and adhere to the already electroded faces of the transducer elements.
A Tonpilz or longitudinal resonator type of transducer is described in U.S. Pat. No. 4,530,138. This type of transducer includes a head mass for projection and/or receipt of acoustic energy, a tail mass operative as an initial element, and active transducer means interposed between and coupled to the head and tail masses. The active transducer means may be a stack of rings of a ceramic piezoelectric having interposed ring electrodes to which electrical connections are made. The electrode may be stamped out of a thin sheet of annealed nickel which is then corrugated such that the front and rear surfaces are serrated to form a plurality of peaks and valleys. When the electrode is placed between two adjacent components such that the peaks on both surfaces of the electrode make intimate contact with the respective components, the valleys on either surface of the electrode define small passageways. Suitable electrical leads are soldered to tabs projecting from the corrugated electrodes. Flowable adhesive is provided to fill the passageways by evacuating a transducer assembly. Heat is then applied to cure a stack of rings and form a ceramic piezoelectric transducer.